1. Field of the Invention
The present invention relates to provide a process for preparing precipitated polyamide powders of highly uniform particle size and low porosity.
2. Discussion of the Background
Polyamide-based coating powders are notable for their high chemical resistance and very good mechanical properties. Among such powders, those obtained by precipitation from ethanolic solution, in accordance, for example, with DE-A 29 06 647, are superior in terms of their processing properties, to those products obtained from a grinding process, in accordance, for example, with DE-A 15 70 392, since precipitation leads to rounder and thus better fluidizable particles. Milled powders as obtained, for example, in accordance with DE-C 28 55 920 possess, moreover, a broad particle size distribution, necessitating a great classification effort. A further advantage of the precipitated powders is the wide scope for variation of the molecular weight (.eta..sub.rel =1.5 to 2.0), whereas milled powders can only be prepared economically at an .eta..sub.rel &lt;1.7.
The process of precipitation in its known form, from alcoholic solutions under pressure, is at its technical limits when it is to be used to prepare compact powders having a low porosity as expressed by their BET surface areas. This applies in particular to fine powders having a particle fraction &lt;100 .mu.m of more than 90%, especially when the fraction d&lt;32 .mu.m, which is responsible for dusting, is difficult to separate off by sieving and screening: this fraction is less in evidence with coarser precipitated powder. If powders having an upper particle size limit of 63 .mu.m, in some cases even &lt;40 .mu.m, are required, they have to be separated off in one or more classifying steps from the precipitated and dried crude powder. The fine fraction &lt;32 .mu.m which is present in the crude powder limits the yield of useful particles in the case of fine powders.
Although it is possible in accordance with DE-A 35 10 689 to obtain fine powders with a highly uniform particle size by using low dissolution temperatures, these products are normally characterized by low bulk densities and high BET surface areas, and therefore do not flow as well. Moreover, the powders are to have a low porosity, as determined by their BET surface area. According to DE-A 44 21 454, precipitated polyamide powders with narrow particle size distributions and low porosity can be prepared. With this process, however, very fine polyamide powders are obtained by a subsequent grinding process. Coarser powders, as are employed for fluidized sinter coating, are to include a high fraction between 60 and 150 .mu.m; in these cases not only the dust fraction but also coarse particles &gt;250 .mu.m have to be separated off by sieving.
There is therefore a need for a modified precipitation process which provides compact powders having a relatively narrow particle size distribution, so that, preferably, classification becomes unnecessary or, at least, it is possible to obtain an increased yield in the course of classification by sieving or screening. Moreover, the powders should have a low porosity as determined by their BET surface areas.
This object has surprisingly been achieved in accordance with the patent claims.